Swine flu? Pffft. Epidemiologists are bracing for a far more lethal bug—one that’ll stop global travel in its tracks.

How it Could Happen

“With a deadly influenza pandemic, it’s a question of when, not if,” says Daniel Barnett, M.D., an instructor at the Johns Hopkins Bloomberg School of Public Health Preparedness Programs. “If we have a 1918-type pandemic, there will be hundreds of millions of deaths globally.” Influenza viruses are profligate gene swappers, meaning they can quickly recombine into more virulent strains that outmaneuver vaccines. One chilling but plausible scenario involves the swine flu virus migrating to the Southern Hemisphere, acquiring genes from a deadlier strain like avian flu, then returning north this fall. “We’ll see this same virus in October in some form,” says legendary epidemiologist Larry Brilliant, M.D., who recently left Google.org to helm the Skoll Urgent Threats Fund, which will tackle pandemics and other major public health issues. “There are lots of reasons to be concerned.”